The present invention relates to a compound engine mount which absorbs vibrations by means of the elasticity of a rubber cushion and the resonance of fluid which flows through an orifice and, more particularly, to a complex engine mount of the type having a variable orifice.
In an automotive vehicle, vibration is developed over a wide range of frequencies and that of amplitudes depending upon the operating conditions of the vehicle, particularly the engine speed. An automotive engine, therefore, is often supported on a vehicle body by compound engine mounts which are capable of absorbing a wide range of vibrations. A compound engine mount includes a first fluid chamber which is surrounded by a rubber cushion and a second fluid chamber which is surrounded by a flexible diaphragm, the first and second fluid chambers being intercommunicated by an orifice. When the rubber cushion is greatly deformed by a load, the fluid is caused to move between the first and second fluid chambers through the orifice to absorb vibrations of large amplitudes. Vibrations of small amplitudes are absorbed by the deformation of the rubber cushion.
Generally, the orifice of such a compound engine mount is provided with fixed dimensions. A problem with an engine mount with a fixed orifice as mentioned is that low frequency vibrations which occur in an idling range of an engine cannot be absorbed unless the effective path area of the orifice is designed small enough to lower the resonance frequency of the fluid, which flows between the fluid chambers. Such would reduce the mass of the fluid inside of the orifice and, thereby, increase the dynamic spring constant, making it difficult for high frequency vibrations whose amplitude is small to be absorbed effectively. On the other hand, should the orifice be provided with a large effective path area in order to maintain a small dynamic spring constant even under high frequency vibrations, the resonance effect of the fluid would not be accomplished under low frequency oscillations.
In the light of the above, there has been proposed to provide fluid communication between the first and second fluid chambers through an orifice which has a relatively large area and is selectively opened and closed by a valve, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 59-40945. The orifice disclosed is closed in response to low frequency vibrations and opened to high frequency vibrations, whereby the dynamic spring constant is increased under low frequency vibrations and decreased under high frequency vibrations. This, however, brings about another problem that the open-close or on-off control of the orifice results in noticeable changes in the dynamic spring constant. Hence, what this type of arrangement can do is simply switching the dynamic spring constant between two different kinds of operating conditions of a vehicle, i.e., special conditions such as rapid accelerating conditions and ordinary operating conditions, and not sequentially varying the dynamic spring constant in response to the change in vibration over the idling to the high speed ranges.
Meanwhile, the engine speed and, therefore, the vibration characteristic varies even in the idling range due to idle-up and others. Hence, the dynamic spring constant should preferably be controlled within the idling range as well. Although an arrangement may be made such that the effective path area of the orifice be continuously controllable in response to the varying engine speed, it cannot be achieved without resorting to a considerably difficult control since, in an idling range, the small area of the orifice should be further controlled.
Another implementation heretofore proposed is an engine mount in which the length of an orifice is controllable in a low frequency, large amplitude vibration range such as during engine cranking, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 60-73146. This kind of scheme is disadvantageous in that because the sectional area of the orifice is too small to noticeably effect the volume of the orifice portion, or the mass of the fluid inside of the orifice portion, and only the damping coefficient is controlled, it is impossible to change the dynamic spring constant in response to an engine speed.